Several methods of producing silica sols have been studied and disclosed in the prior art. Preparation of dilute silica sols from silicate and acid is described in Alexander et al. U.S. Pat. No. 2,601,235. Example 2 of U.S. Pat. No. 2,601,235 describes a process by which sodium silicate and sulfuric acid are reacted in the presence of silica nuclei. By controlling the reaction conditions, newly formed silica is deposited on the nuclei present. The existing silica particles grow in size and no new nuclei form. The colloidal silica particles produced in this manner are relatively uniform in size and are dense. However, this Example has as its object a precipitated silica; no provision is made for removal of the reaction byproduct, sodium sulfate, while maintaining the sol in a stable condition.
U.S. Pat. No. 2,605,228 also teaches the reaction of sodium silicate and acid to produce colloidal silica. In this process no nuclei are present and, under the conditions described, the silica formed is sufficiently porous to contain occluded sodium ions. As in the process previously described, the reaction byproduct is a sodium salt which is not removed until the silica in the sol is precipitated. Neither process teaches a method of removing salt from a stable silica sol.
Ion exchange resins have been used to remove salts from silica sols; alternatively, silica sols can be made without a salt as a byproduct by the reaction of metallic silicon and water. (See Balthis, U.S. Pat. No. 2,614,995.) The use of metallic silicon or ion exchange resins to obtain saltfree, concentrated silica sols involves significant expense. Further, the sols produced by these methods contain porous, sponge-like silica particles. Achieving highly concentrated sols of these porous particles is extremely difficult. (See Iler, Colloid Chemistry of Silica and Silicates, p. 98, Cornell University Press, 1955.)
The process of this invention provides a path to stable concentrated silica sols without requiring the use of expensive ion exchange resins. Microporous membranes are used in this process to separate water and soluble salts from silica sols. The use of such microporous membrane filters is well known for several processes, for example, separation of proteins from blood plasma. Microporous membrane filter, also called ultrafilters, are commercially available in a number of pore sizes.
Chilton, U.S. Pat. No. 3,560,400 describes the removal of water from a dilute silica sol by ultrafiltration. However, the reference states that poor results are achieved by filtering sols of pH 9 to 11 and states that improved results are obtained if the sol is treated with a cation exchange resin to lower the pH to 2-4. Thus, Chilton teaches removal of water but not salts from sols. Further, Chilton does not deal with the problems associated with silica sols having a basic pH. In particular, Chilton does not teach how to avoid the formation of aggregates of colloidal silica particles which results in a rapidly increasing viscosity of the sols prior to obtaining the desired concentration. Such silica aggregates are deposited on the ultrafilter membrane, reducing the filtration rate to an impractically low value, and the membrane must be repeatedly cleaned. It is not practical to operate an ultrafilter under such adverse conditions. This invention provides a process having conditions within which microporous membrane filters can be effectively used to remove water and salts from a silica sol having a basic pH.